1. Field of the Invention
The present invention relates to a method and apparatus for positioning a specimen on a specimen carrier (e.g., 3 mm grid or post) such that the specimen may be further thinned with e.g., FIB milling, broad ion beam milling, or laser ablation. In addition, this method and apparatus allows for positioning of the specimen to reduce or eliminate ion milling or laser ablation curtaining artifacts.
2. Description of the Related Art
The ex-situ lift-out (EXLO) method is a well-known technique that is typically used to prepare specimens for subsequent transmission electron microscope (TEM) or other analysis using focused ion beam (FIB) milling routines. In this method, specimens are completely FIB milled free inside of a charged particle vacuum environment and then the specimen is manipulated to a carbon or formvar coated TEM grid using a microscope and micromanipulator system in ambient conditions outside of the FIB. The advantages to the EXLO method are: (i) little or no initial specimen preparation is needed, (ii) it is site specific, (iii) it is fast, and (iv) it has a high success rate. The primary disadvantages to the EXLO technique are: (i) it is difficult and/or impossible to further thin the specimen, (ii) it is difficult and/or impossible to perform back-side milling on a specimen to avoid curtaining artifacts, and (iii) the carbon or formvar support film may inhibit certain analyses or cleaning operations.
Manipulation in ambient conditions of the specimen to the grid may be performed by different probing methods. One conventional method utilizes static attraction by touching a solid glass needle to the specimen for transfer to a carbon or formvar coated grid. In another method, a needle can be dipped in glue to adhere the specimen to the probe for transfer to a grid which also contains glue. In yet another method of specimen manipulation technique, a suction or vacuum pulled through a hollow needle can be used to capture the specimen for manipulation to a carbon or formvar grid. Probes with a variety of grippers may also be used to transfer the specimen to a grid.
FIB milling or laser ablation may be used to create a cross sectioned surface for subsequent site specific analytical characterization. It is well known that material removal rates are dependent on dose, incidence angle, crystal orientation, and material composition. A ubiquitous cross sectioning artifact known colloquially as “curtaining” or the “waterfall effect” consists of local roughness and thickness variation of the surface, and is a direct result of differences in removal rates that may be inherent to the specimen composition or geometry. Curtaining is so named due to the appearance and observation of lines of differential milling that resemble theater curtains which form on the milled surface parallel to the beam direction. The presence and observation of these lines is a direct indication of an uneven and rough milled surface.
Surface roughness and irregular specimen thickness can be problematic for many electron microscopy and other techniques used to analyze the FIB milled surface. As an example, conventionally prepared FIB milled specimens of semiconductor gate structures will yield curtaining artifacts that create thickness changes in the substrate which render 2D dopant analysis via electron holography useless.
Accordingly, the need remains for a method to eliminate such irregularities in the surface of specimens so that they can be properly analyzed.